OCR Text |
Show stage which was converted to NO in the second stage (based on NO measured in the furnace at a distance of 3.365 m from the top). Data for these three fuels (oil, 30% doped nitrotoluene in toluene and 100% nitrotoluene) and first stage stoichiometries between 0.5 and 0.8 have been included. As the TFN concentration increased (due to decreasing SR1), the percent conversion decreased. This behaviour is analogous to that reported previously for poorly mixed excess air diffusion flames (3), with fuel nitrogen content less than 1%. The second stage burnout can be con sidered an excess air flame with a gaseous hydrocarbon fuel. The general conversion decrease may be the result of a competition between N2 formation reactions which are second order in XNi and first order NO formation reactions. DISCUSSION OF RESULTS o nr-------------------~ Z .10 2 c· a) ~ 26 ~ 24 c o u 22 • ~~~----~----~--~~ 310 no 1lO l40 lSO 360 3J1O *IT--~~------------~ y-S,82'4e·<4-"'- I ,I089 A-I ,OO o Z 7 -o , z x. 5 c) .! • r! ~ 3 c: o u 2 • toGO 2000 lOGO 4100 y -35,9928 • "'-02787 A- 0,98 • Io ~---5-000- ---10~000- ---150-00 ---~ ppat XNt (Ory, 01 02) Figure 8. Conversion of TFN to exhaust NO. NOx control via staged combustion involves optimizing the first stage of the combustor with respect to 1) stoichiometry, and to 2) residence time or length of the primary zone. Optimization of first stageichiometry The optimization of first stage stoichiometry is more complex. Measurements within the fuel-rich first stage show that at the optimum stoichiometry NO, HCN and NH~ co-exist. TFN exists mainly as HCN and NH3 for more fuel-r1ch conditions. For conditions less fuel-rich than the optimum, NO is the primary fixed nitrogen species. Three factors influence the stoichiometry at which the minimum- exhaust NO emissions will occur: 1 the amount of TFN species present, 2 the relative conversion efficiencies of the TFN species to NO in the second stage flame, and 3 the influence of other partial combustion products on TFN conversion. Second stage TFN conversion decreases as the TFN distribution shifts in favour of HeN and NH3 (and as the hydrocarbon content of the primary stage reactants increases) (4). Figure 9 shows the percent conversion of the first stage TFN for three fuels (oil, 30NT and lOON). As the first stage stoichiometry is progressively reduced from SR1=1.1, the TFN decreases (due to decreased first stage NO formation) and the percent conversion also decreases (due to increased second stage combustion). These two effects result in a rapid decrease in exhaust NO. Below SRI=0.9 for oil and lOON, the 6 |